This is an Astronomy Glossary made up of the series Astronomy Term of the Day on the Facebook Page of The Secrets of the Universe.

Click the Respective Letter to jump to that section of the Glossary.

A | B | C | D | E | F | G | H | I | J | K | L | M | N| O | P | Q | R | S | T | U | V | W | X | Y | Z


Absolute Magnitude: In astronomy, Absolute magnitude is a measure of the luminosity of a celestial object, on a logarithmic astronomical magnitude scale. An object’s absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs (32.6 light-years).

Absolute Zero: The lowest temperature possible, equivalent to -273.150C (or 0 on the absolute Kelvin scale), at which point atoms cease to move altogether and molecular energy is minimal.

Albedo: Albedo (meaning ‘whiteness’) is the measure of the diffuse reflection of solar radiation out of the total solar radiation received by an astronomical body. It is dimensionless and measured on a scale from 0 (Black Body) to 1 (White Body).

Albedo: Albedo (meaning ‘whiteness’) is the measure of the diffuse reflection of solar radiation out of the total solar radiation received by an astronomical body. It is dimensionless and measured on a scale from 0 (Black Body) to 1 (White Body).

Angular Size and Distance: The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°.

Antimatter: Antimatter is the opposite of normal matter. More specifically, the sub-atomic particles of antimatter have properties opposite those of normal matter. The particles have reversed electric charge.

Aperture: The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).

Apparent Magnitude: The apparent magnitude of an astronomical object is a number that is a measure of its brightness as seen by an observer on Earth.

Apsis: An apsis is either of the points in the orbit of a celestial object that is farthest from or closest to the body being orbited. The closest point is the periapsis and the farthest is called the apoapsis. The line joining these two points is called the line’ of apsides and is another name of the major axis of the orbit: the ellipse.

Asterism: Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper.

Asteroid (Minor Planet): A solid body orbiting the Sun that consists of metal and rock. Most are only a few miles in diameter and are found between the orbits of Mars and Jupiter, too small and far away to be seen easily in a small telescope. A few venture closer to the Sun and cross Earth’s orbit.

Astronomical Unit: The average distance from Earth to the Sun, slightly less than 93 million miles.

Astroseismology: A very important branch of astronomy that focuses on the study of the acoustic vibrations of stars in order to learn more about its internal structure.

Averted Vision: Viewing an object by looking slightly to its side. This technique can help you detect faint objects that are invisible when you stare directly at them.

Axions: Hypothetical particles that are the constituents of the dark matter. They must have 0 electric charge and 0 intrinsic spin.


Balmer Series: The Balmer series is the name given to a series of spectral emission lines of the hydrogen atom that result from electron transitions from higher levels down to the energy level with principal quantum number 2.

Barlow Lens: A lens that’s placed into the focusing tube to effectively double or triple a telescope’s focal length and, in turn, the magnification of any eyepiece used with it.

Barium Stars: In astronomy, Barium stars are regarded spectral class G to K giants, whose spectra indicate an overabundance of s-process elements by the presence of singly ionized barium, Ba II, at ? 455.4 nm. Barium stars are known to be rich in Carbon

Barycenter: The barycenter is the center of mass of the bodies that are orbiting each other. In the case of the earth-sun system, the barycenter lies inside the sun. So to an outside observer, the earth appears to revolve around the sun while the sun wobbles around the barycenter.

Baryons: In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks. Baryons belong to the hadron family of particles, which are the quark-based particles.

Big Bang: It is a cosmological model which describes the beginning of the universe and its growth into a large scale structure. Its emphasis on the consideration that the universe started from a state of high density and temperature and eventually expanded out. The discovery of CMB radiation is crucial evidence of the big bang.

Big Crunch: The Big Crunch is one of the theoretical scenarios for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach zero or causing a reformation of the universe starting with another Big Bang.

Big Freeze: The Big Freeze, is an idea of an ultimate fate of the universe in which the universe has evolved to a state of no thermodynamic free energy and therefore can no longer sustain processes that increase entropy.

Binding energy: The minimum energy required to disassemble a system of particles into separate parts. This energy is equal to the mass defect minus the amount of energy, or mass, that is released when a bound system is created and is what keeps the system together.

Black Body: In astronomy, A black body is theorized as an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.

Black Dwarf: A theoretical endpoint of the stellar evolution, especially that of a white dwarf. When a sun-like star ends its life, it becomes a white dwarf star. A black dwarf is formed when a white dwarf cools to the extent that it can no longer shine. Mathematically, trillion years are needed for its formation, indicating that there are no black dwarfs in the universe at the moment.

Black Hole: A concentration of mass so dense that nothing — not even light — can escape its gravitational pull once swallowed up. Many galaxies (including ours) have supermassive black holes at their centers.

Blue Moon: Traditionally, something that happens rarely or never. More recently, this has come to mean the second full Moon in a single calendar month.

Bolometric Corrections: In astronomy, the bolometric correction is the correction made to the absolute magnitude of an object in order to convert its visible magnitude to its bolometric magnitude. It is large for stars which radiate much of their energy outside of the visible range.

Bosons: In quantum mechanics, a boson is a particle that follows Bose-Einstein statistics. Bosons make up one of the two classes of particles, the other being fermions.

Brown Dwarf: A celestial object intermediate in size between a giant planet and a small star, believed to emit mainly infrared radiation.


Carbon Detonation: Carbon detonation is the violent reignition of thermonuclear fusion in a white dwarf star that was previously slowly cooling. It involves a runaway thermonuclear process producing a Type Ia supernova which releases an immense amount of energy as the star is blown apart.

Celestial Coordinates: A grid system for locating things in the sky. It is known as one of the most important concepts in astronomy. It’s anchored to the celestial poles (directly above Earth’s north and south poles) and the celestial equator (directly above Earth’s equator). Declination and right ascension are the celestial equivalents of latitude and longitude.

Cepheid Variable: Cepheid variable stars are intrinsic variables which pulsate in a predictable way. In addition, a Cepheid star’s period (how often it pulsates) is directly related to its luminosity or brightness. Cepheid variables are extremely luminous and very distant ones can be observed and measured.

Chandrasekhar Limit: In Astronomy, the Chandrasekhar limit is the maximum mass of a stable white dwarf star. Beyond this limit, the star will collapse to become a neutron star or a black hole. Actually, a  white dwarf is stable against gravitational collapse by electron degeneracy pressure. Above this limit, the stability ends and this results into a supernova.

Chromosphere: The chromosphere is the second of the three major layers of the solar atmosphere which sits just above the Photosphere. This layer is 3000 to 5000 km deep and its density is very low which makes it visible only during the total solar eclipse, in a reddish hue. The temperature at the base is about 4,400 K and it mysteriously increases with the increasing height.

Circumpolar: Denotes an object near a celestial pole that never dips below the horizon as Earth rotates and thus does not rise or set.

Clock Drive: In astronomy, a clock drive (also known as a field rotator) is a motor-controlled mechanism used to move an equatorial mount telescope along one axis to keep the aim in exact sync with the apparent motion of the fixed stars on the celestial sphere.

Cocoon Star: A Cocoon Star is a star that is a strong source of infrared waves but is invisible at optical wavelength because the star is shrouded in a dense cloud of dust and gas. Such stars are a few million years old and are hidden in the material that is left over after their formation.

CMB Radiation: The Cosmic Microwave Background Radiation is the leftover radiation from the time of the recombination era. Its temperature is around 2.725 Kelvin and serves as a piece of strong evidence for Big Bang.

Collimation: Aligning the optical elements of a telescope so that they all point in the proper direction. Most reflectors and compound telescopes require occasional collimation in order to produce the best possible images.

Color Index: In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The smaller the color index, the bluer (or hotter) the object is.

Comet: A comet is a “dirty snowball” of ice and rocky debris, typically a few miles across, that orbits the Sun in a long ellipse. When close to the Sun, the warmth evaporates the ice in the nucleus to form a coma (cloud of gas) and a tail. Named for their discoverers, comets sometimes make return visits after as little as a few years or as long as tens of thousands of years.

Compact Star: A compact star is a term collectively used for the white dwarfs, neutron stars, and the black holes.

Compound Telescope: A telescope with a mirror in the back and a lens in the front. The most popular designs are the Schmidt-Cassegrain telescope (SCT) and the Maksutov-Cassegrain telescope (commonly called a “Mak”).

Conjunction: When the Moon or a planet appears especially close either to another planet or to a bright star.

Constellation: In astronomy, a constellation is a distinctive pattern of stars used informally to organize a part of the sky. There are 88 official constellations, which technically define sections of the sky rather than collections of specific stars.

Convective Overshoot: Stellar interior comprises 3 main regions: The core, radiation zone, and convection zone. Usually, the energy is transported through the convection currents. In convective overshoot, the convection currents penetrate deeper into the radiation zone and cause the heavy elements to move from the core to the other regions of the star.

Cosmic Inflation: It was the time period after the Big Bang in which the universe underwent an exponential growth in its size. It lasted from 10-37 sec to 10-33 sec.

Cosmic voids: Voids are the large regions of the universe containing few or no galaxies and can take up 98% space of the universe. The first of these voids discovered was in the constellation of Bootes in 1981. It occupies 2% of the total universe.

Culmination: The moment when a celestial object crosses the meridian and is thus at its highest above the horizon.


Dark Adaptation: The eyes’ transition to night vision, in order to see faint objects. So dark adaptation is rapid during the first 5 or 10 minutes after you leave a well-lit room, but full adaptation requires at least a half-hour — and it can be ruined by a momentary glance at a bright light.

Dark Matter: It is a hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe and about a quarter of its total energy density. The majority of dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet-undiscovered subatomic particles.

Dark Energy: A theoretical form of energy postulated to act in opposition to gravity and to occupy the entire universe, accounting for most of the energy in it and causing its expansion to accelerate.

Declination (Dec.): The celestial equivalent of latitude in astronomy, denoting how far (in degrees) an object in the sky lies north or south of the celestial equator.

Degenerate Matter: Degenerate matter is a highly dense state of fermionic matter in which particles must occupy high states of kinetic energy to satisfy the Pauli exclusion principle. The description applies to matter composed of electrons, protons, neutrons or other fermions.

Dobsonian (“Dob”): A type of Newtonian reflector, made popular by amateur astronomer John Dobson, that uses a simple but highly effective wooden mount. Dobs provide more aperture per dollar than any other telescope design.

Doppler Effect: The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source.

Double Star (Binary Star): Two stars that lie very close to, and are often orbiting, each other. Line-of-sight doubles are a consequence of perspective and aren’t physically related. Many stars are multiples (doubles, triples, or more) gravitationally bound together. Usually, such stars orbit so closely that they appear as a single point of light even when viewed through professional telescopes.

Dredge Up: A dredge-up is a period in the evolution of a star where a surface convection zone extends down to the layers where the material has undergone nuclear fusion. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere where they can appear in the spectrum of the star.


Earthshine: Sunlight reflected by Earth that makes the otherwise dark part of the Moonglow faintly. It’s especially obvious during the Moon’s thin crescent phases.

Eccentricity: The measure of how much an orbit deviates from being circular.

Eclipse: An event predicted by astronomy, that occurs when the shadow of a planet or moon falls upon a second body. A solar eclipse occurs when the Moon’s shadow falls upon Earth, which we see as the Moon blocking the Sun. When Earth’s shadow falls upon the Moon, it causes a lunar eclipse.

Ecliptic: The path among the stars traced by the Sun throughout the year. The Moon and planets never stray far from the ecliptic.

ED Pressure: The Electron Degeneracy Pressure (EPD) is a direct result of Pauli’s exclusion principle of quantum mechanics. No 2 electrons can occupy the same quantum state. So if the matter is compressed into smaller volumes of space, it results in emergent pressure against compression.

Eddington Limit: The maximum luminosity that a body can achieve such that there is a balance between the outward radiation pressure and inward gravitational collapse. Most stars do not reach this limit and it is mostly used to explain the tremendous luminosity of black holes and quasars.

Ellipticity: In astronomy, ellipticity is the measure of the amount by which an object such as a planet or a galaxy, deviates from a perfect sphere, also known as oblateness. It is typically an indication of how fast a body is rotating.

Elongation: The angular distance the Moon or a planet is from the Sun. The inner planets of Mercury and Venus are best seen when at maximum elongation, and thus are highest above the horizon before sunrise or after sunset.

Emission line: It is a spectral line that we get when an electron, atom or molecule makes a transition from higher energy state to lower energy state. Every element has its own, very specific spectrum of the emission line. This constitutes a major portion in the field of astronomy.

Entropy: A thermodynamic quantity representing the unavailability of a system’s thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system.

Ephemeris: A timetable with celestial coordinates that indicates where a planet, comet, or other body moving in relation to background stars will be in the sky. Its plural is ephemerides (pronounced eff-uh-MEHR-ih-deez).

Equinox: The two times each year, near March 20th and September 22nd, when the Sun is directly overhead at noon as seen from Earth’s equator. On an equinox date, day and night are of equal length.

Equivalence Principle: It states that the gravitational mass of a body equals its inertial mass. So a person sitting in a windowless room cannot distinguish b/w being on earth or in a spaceship in deep space accelerating at 9.81 m/s2. This has applications in astrophysics, particularly in explaining the gravitational red-shift.

Escape Velocity: The minimum speed needed for a free object to escape from the gravitational influence of a massive body.

Eternally Collapsing Object (ECO): An ECO is a self-gravitating ball of ultra-hot plasma that is as compact as a black hole but qualitatively different from the true black hole. The term was first coined by Indian astrophysicist Abhas Mitra in 1998 and still remains unchallenged.

Event Horizon: In stellar astrophysics, the event horizon is defined as a boundary from within which it is impossible for the matter and energy to escape the gravitational pull of a black hole. It is where the escape velocity is greater than that of light.

Eyepiece: The part of a telescope that you look into. A telescope’s magnification can be changed by using eyepieces with different focal lengths; shorter focal lengths yield higher magnifications. Most eyepieces have metal barrels that are 1¼ inches in diameter; other standard sizes are 0.965 and 2 inches across.

Exoplanets: An exoplanet or extrasolar planet is a planet outside the Solar System.

Exotic Star: An exotic star is a hypothetical star that is composed of exotic matter other than the protons, electrons, neutrons, and muons. So this includes quark star, preon stars, the boson star, and the plank star. All these stars are theoretical and no evidence has even been observed.


Fermions: In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. These particles obey the Pauli exclusion principle. Fermions include all quarks and leptons, as well as all composite particles made of an odd number of these, such as all baryons and many atoms and nuclei.

Fermi Bubbles: Fermi bubbles are two gamma and X-ray bubbles that extend 25k light years above and below the plane of the milky way.

Field of View: The circle of the sky that you see when you look through a telescope or binoculars. Generally, the lower the magnification, the wider the field of view.

Finderscope: A small telescope used to aim your main scope at an object in the sky. Finderscopes have low magnifications, wide fields of view, and (usually) crosshairs marking the center of the field.

Flare Stars: A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in the stars’ atmospheres.

Focal Length: The distance (usually expressed in millimeters) from a mirror or lens to the image that it forms. In most telescopes, the focal length is roughly equal to the length of the tube. Some telescopes use extra lenses and/or mirrors to create a long effective focal length in a short tube.

Focal Ratio (f/number): A lens or mirror’s focal length divided by its aperture. For instance, a telescope with an 80-mm-wide lens and a 400-mm focal length has a focal ratio of f/5.

Fusor: A celestial body that, through self-gravity, is able to perform nuclear fusion within its core, at any point in its life.


Galactic Year: The galactic year, also known as a cosmic year, is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy. Estimates of the length of one orbit range from 225 to 250 million terrestrial years.

Galaxy: A vast collection of stars, gas, and dust, typically 10,000 to 100,000 light-years in diameter and containing billions of stars (from galaxias kuklos, Greek for “circle of milk,” originally used to describe our own Milky Way).

Gamma Waves: The most energetic and shortest wavelength type of EM radiation.

Gibbous: When the Moon or other body appears more than half, but not fully, illuminated (from gibbus, Latin for “hump”).

Gluons: A gluon is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles.

Granules: Loos at a very detailed image of the sun. You will notice that the surface is covered with small “cells”. These are called granules and are formed by the convection zone beneath.

Gravastar: A theoretical alternative to a black hole resulting from real and physical limitations on the formation of a black hole. It comprises a central vacuum bubble having dark energy and around this bubble, a thin layer of Bose-Einstein condensate.

Gravitational Instability: Gravitational Instability is a situation when an object’s self-gravity exceeds opposing forces such as outward gas pressure. White dwarfs, Neutron stars, Black Holes all are formed due to gravitational collapse of a star when they run out of fusion material.

Graviton: In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravity.

Gravity Assist: Gravitational assist is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically to save propellant.


H-? (Alpha) Line: The H – ? line is the red visible spectral line created by a hydrogen atom when an electron falls from 3rd lowest to 2nd lowest energy level. This line corresponds to the wavelength of 656.28 nm. The presence of this line in the spectrum of any celestial object such as nebulae is evidence that the hydrogen there is ionized. H – ? line is also useful in observing the solar prominences and deep sky objects.

H I Region: An HI region or H I region is a cloud in the interstellar medium composed of neutral atomic hydrogen, in addition to the local abundance of helium and other elements. These regions do not emit detectable visible light but are observed by the 21-cm region spectral line.

H II Region: A region of interstellar atomic hydrogen that is ionized. It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm.

Hadrons: In particle physics, a hadron is a composite particle made of two or more quarks held together by the strong force in a similar way as molecules are held together by the electromagnetic force. Most of the mass of ordinary matter comes from two hadrons, the proton, and the neutron.

Hawking Radiation: Hawking radiation is black-body radiation that is predicted to be released by the black holes near the event horizon. Hawking radiation is speculative and there is no experimental proof of it yet.

Hayashi Track: The Hayashi track is a luminosity-temperature relationship obeyed by infant stars of less than 3 Solar Masses in the pre-main-sequence phase (PMS phase) of stellar evolution. It is named after Japanese astrophysicist Chushiro Hayashi.

Heat Death: The heat death of the universe, also known as the Big Chill or Big Freeze, is an idea of an ultimate fate of the universe in which the universe has evolved to a state of no thermodynamic free energy and therefore can no longer sustain processes that increase entropy.

Heliopause: The theoretical boundary where the Sun’s solar wind is stopped by the interstellar medium. The heliopause is about 123 astronomical units (AU) or 18 billion km [11 billion miles] from the Sun.

Helium Flash: A helium flash is the onset of runaway helium burning in the core of mid-sized star such as the sun. After hydrogen fusion has ceased, the core is made of inert helium. This results in an upsurge of temperature, igniting helium fusion with an explosion.

HH objects: Herbig–Haro (HH) objects are bright patches of nebulosity associated with newborn stars. They are formed when narrow jets of partially ionized gas ejected by said stars collide with nearby clouds of gas and dust at speeds of several hundred kilometers per second.

Higgs field: The Higgs field is a field of energy that is thought to exist in every region of the universe. The field is accompanied by a fundamental particle known as the Higgs boson, which is used by the field to continuously interact with other particles, such as the electron.

Higgs Boson: The Higgs boson is an elementary particle in the Standard Model of particle physics, produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory.

Histogram: A plot of the number of pixels in an image at each brightness level. It’s a useful tool for determining the optimum exposure time; the histogram of a properly exposed image generally peaks near the middle of the available brightness range and falls to zero before reaching either end.

Hot Jupiter: Hot Jupiters are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (P<10 days). They have close proximity to their stars and high surface-atmosphere temperatures.

Hot Neptune: A hot Neptune or Hoptune is a type of giant planet with a mass similar to that of Uranus or Neptune orbiting close to its star, normally within less than 1 AU.

HR Diagram: The Hertzsprung–Russell diagram is a scatter plot of stars showing the relationship between the stars’ absolute magnitudes or luminosity versus their stellar classifications or effective temperatures. Learn more about it here.

Hubble’s Law: A law stating that the redshifts in the spectra of distant galaxies (and hence their speeds of recession) are proportional to their distance.

Hypergiants: They are the most massive and luminous stars known in the universe. When these stars started their life, they were massive, somewhat around 25 times the mass of the sun. They burn their fuel quickly and are short-lived.


Inclination: The angle between the plane of an orbit and a reference plane. For example, NASA satellites typically have orbits inclined 28° to Earth’s equator.

Interacting Binary: An Interacting binary star is a type of binary star in which one or both of the component stars has filled or exceeded its Roche lobe. When this happens, material from one star (the donor star) will flow towards the other star (the accretor).

Intra-Cluster Medium: In astronomy, the intracluster medium is the superheated plasma that permeates a galaxy cluster. The gas consists mainly of ionized hydrogen and helium and accounts for most of the baryonic material in galaxy clusters.


Jansky: The jansky is a non-SI unit of spectral flux density, or spectral irradiance, used especially in radio astronomy. It is equivalent to 10?26 watts per square meter per Hertz.

Jeans Instability: In stellar physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation, named after James Jeans. It occurs when the internal gas pressure is not strong enough to prevent the gravitational collapse of a region filled with matter.

Jeans Length: In Astrophysics, Jeans’ length is the critical radius of a cloud (typically a cloud of interstellar dust) where thermal energy, which causes the cloud to expand, is counteracted by gravity, which causes the cloud to collapse.

Jovian Mass: Jupiter mass, also called Jovian mass, is the unit of mass equal to the total mass of the planet Jupiter. This value may refer to the mass of the planet alone, or the mass of the entire Jovian system to include the moons of Jupiter. It is represented by MJ.

Julian year: In astronomy, a Julian year is a unit of measurement of time defined as exactly 365.25 days of 86400 SI seconds each.


Kaon: In particle physics, a kaon also called a K meson and denoted K, is any of a group of four mesons distinguished by a quantum number called strangeness. In the quark model, they are understood to be bound states of a strange quark and an up or down antiquark.

KH Mechanism: The Kelvin–Helmholtz mechanism is an astronomical process that occurs when the surface of a star or a planet cools. The cooling causes the pressure to drop, and the star or planet shrinks as a result. This mechanism is evident on Jupiter and Saturn and on brown dwarfs which cannot undergo Nuclear Fusion.

Kuiper Belt: The Kuiper belt is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20 to 200 times as massive.


Lagrange point: A Lagrange point is a location in space where the combined gravitational forces of two large bodies, such as Earth and the sun or Earth and the moon, equal the centrifugal force felt by a much smaller third body.

Lepton: In particle physics, a lepton is an elementary particle of half-integer spin (spin ?1?2) that does not undergo strong interactions.

Local Bubble: The local bubble is a large cavity in the Interstellar Medium (ISM) in the Orion arm of the Milky Way. It is about 300 ly across in which the Sun and other nearby stars reside. The average density of neutral hydrogen atoms is 10 times lower than that of ISM. The bubble was formed due to a nearby supernova explosion, some 20 million years ago, which pushed the dust and gas aside in the ISM.

Local Group: The Local Group is just one collection of galaxies in the even bigger Virgo Supercluster. The largest, most massive galaxies in the Local Group are the Milky Way, Andromeda and the Triangulum Galaxy. Each of these galaxies has a collection of satellite galaxies surrounding them.

Libration: A slight tipping and tilting of the Moon from week to week that brings various features along the limb into better view. The main causes are two aspects of the Moon’s orbit: its elliptical shape and inclination to the ecliptic.

Light-year: The distance that light (moving at about 186,000 miles per second) travels in one year, or about 6 trillion miles.

Light Pollution: A glow in the night sky or around your observing site caused by artificial light. It greatly reduces how many stars you can see. Special light-pollution filters can be used with your telescope to improve the visibility of celestial objects.

Limb: The edge of a celestial object’s visible disk.

Limb Darkening: Limb darkening is an optical effect seen in stars (including the Sun), where the center part of the disk appears brighter than the edge or limb of the image.

Luminosity: In astronomy, luminosity is the total amount of energy emitted per unit of time by a star, galaxy, or other astronomical objects. In SI units luminosity is measured in joules per second or watts.


Magellanic Clouds: Magellanic Clouds are the two satellite galaxies of our own Milky Way which are visible from the southern hemisphere as misty patches in the night sky. They are named after the Portuguese explorer, Ferdinand Magellan, who described them in his voyage around the globe. The large and small Magellanic clouds are 10,000 light years apart and are destined to merge as a single galaxy in the future.

Magnetar: A magnetar is a type of neutron star believed to have an extremely powerful magnetic field. The magnetic field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.

Magnification (power): The amount that a telescope enlarges its subject. It’s equal to the telescope’s focal length divided by the eyepiece’s focal length.

Magnitude: A number denoting the brightness of a star or other celestial object. The higher the magnitude, the fainter the object. For example, a 1st-magnitude star is 100 times brighter than a 6th-magnitude star.

Main Sequence Star: The Stars which fuse hydrogen atoms in their core to form helium atoms are main-sequence stars. 90% stars, including our Sun, are main-sequence stars. These stars are in complete hydrostatic equilibrium. This was the First Astronomy Term of the Day to be published on the page!

Meridian: The imaginary north-south line that passes directly overhead (through the zenith).

Messier object: An entry in a catalog of 103-star clusters, nebulas, and galaxies compiled by French comet hunter Charles Messier (mess-YAY) between 1758 and 1782. The modern-day Messier catalog contains 109 objects.

Metallicity: In astronomy, metallicity is used to describe the abundance of elements present in an object that is heavier than hydrogen or helium. Most of the physical matter in the Universe is in the form of hydrogen and helium, so astronomers use the word “metals” as a convenient short term for “all elements except hydrogen and helium”.

Meteor: A brief streak of light caused by a small piece of solid matter entering Earth’s atmosphere at tremendous speed (typically 20 to 40 miles per second). Also called a “shooting star.” If material survives the trip through the atmosphere, it’s called a meteorite after landing on Earth’s surface.

Meteor Shower: An increase in meteor activity at certain times of the year due to Earth passing through a stream of particles along a comet’s orbit around the Sun.

Microquasar: A microquasar is an X-ray binary system which launches and collimates relativistic jets. These binary systems are formed by a normal star and a compact object, a black hole or a neutron star, which accretes mass from the star.

Milky Way: A broad, faintly glowing band stretching across the night sky, composed of billions of stars in our galaxy too faint to be seen individually. It’s invisible when the sky is lit up by artificial light or bright moonlight.

Minkowski Space: It is the combination of the 3-dimensional Euclidean space and time into a 4-D continuum. It represents a flat universe devoid of any curvature. So this means that no gravity is present in such spaces and hence act as the basic framework for special relativity.

MK System: MK system stands for the Morgan Keenan system. Using this, astronomers classify stars using the alphabets O, B, A, F, G, K, and M.

Mira Variables: Mira variables, named for the prototype star Mira, are a class of pulsating variable stars characterized by very red colors, pulsation periods longer than 100 days, and amplitudes greater than one magnitude in infrared and 2.5 magnitudes at visual wavelengths.

Monopole: The magnetic monopole is a hypothetical particle that is an isolated magnet with a single pole. This particle is hypothesized by Grand Unified Theory (GUT). However, no monopoles have been discovered or invented yet.

Mount: The device that supports your telescope, allows it to point to different parts of the sky, and lets you track objects as Earth rotates. A sturdy, vibration-free mount is every bit as important as the telescope’s optics. A mount’s top, or head, can be either alt-azimuth (turning side to side, up and down) or equatorial (turning parallel to the celestial coordinate system). “Go-To” mounts contain computers that can find and track celestial objects automatically once the mounts have been aligned properly.

Muons: The muon is an elementary particle similar to the electron, with an electric charge of ?1 e and a spin of 1/2, but with a much greater mass. It is classified as a lepton with a mean lifetime of 2.2 ?s.


Nebula: Latin for “cloud.” Bright nebulas are great clouds of glowing gas, lit up by stars inside or nearby. Dark nebulas are not lit up and are visible only because they block the light of stars behind them.

Neutrino: Neutrino is a fermion that interacts only via the weak subatomic force and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small that it was long thought to be zero.

Neutron Star: A celestial object of the very small radius (typically 30 km) and very high density, composed predominantly of closely packed neutrons. Neutron stars are thought to form by the gravitational collapse of the remnant of a massive star after a supernova explosion, provided that the star is insufficiently massive to produce a black hole.

NGC Catalogue: The New General Catalogue of Nebulae and Clusters of Stars is a catalog of deep-sky objects compiled by John Louis Emil Dreyer in 1888.


Objective: A telescope’s main light-gathering lens or mirror.

Occultation: When the Moon or a planet passes directly in front of a more distant planet or star. A grazing occultation occurs if the background body is never completely hidden from the observer.

Oort Cloud: The Oort Cloud is an extended shell of icy objects that exist in the outermost reaches of the solar system. The Oort Cloud is roughly spherical and is thought to be the origin of most of the long-period comets that have been observed.

Opacity: Opacity is the quantity that measures a material’s ability to absorb or scatter electromagnetic radiation. It increases with increasing proportions of heavier elements in the nucleus of the star.

Open Clusters: An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy.

Optical Depth: The optical depth is the measure of how much light is absorbed in traveling through a medium, such as the atmosphere of a star. Radio waves are unaffected by the dust and hence their optical depth is zero.

Oppenheimer- Volkoff Limit: In stellar astrophysics, the Oppenheimer Volkoff limit is the upper limit of mass of a neutron star beyond which it must collapse to become a black hole or possibly a quark star.

Opposition: When a planet or asteroid is opposite the Sun in the sky. At such times the object is visible all night — rising at sunset and setting at sunrise.

Orbital Eccentricity: The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptic orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola.

Orbital resonance: In Astronomy, Orbital Resonance occurs when 2 orbiting bodies exert a periodic gravitational force on each other just because their orbital periods are related to each other by a ratio of two small numbers.


Parallax: The apparent offset of a foreground object against the background when your perspective changes. At a given instant, the Moon appears among different stars for observers at widely separated locations on Earth. Astronomers directly calculate the distance to a nearby star by measuring its incredibly small positional changes (its parallax) as Earth orbits the Sun.

Parsec: In Astronomy, a parsec is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arc-second.

Pauli’s Principle: The Pauli’s exclusion principle states that no 2 identical fermions (Particles with 1/2 integer spin) can occupy the same quantum state. Aside from its applications in the quantum model, it also helps in explaining the stability of white dwarfs and neutron stars.

Phase: The fraction of the Moon or other bodies that we see illuminated by sunlight.

Photosphere: Photosphere is a star’s outer shell from which light is radiated. Its the innermost layer of the stellar atmosphere to which our eyes can penetrate through the superficial transparent layers. Our view is obstructed beyond this layer by the rapidly increasing opacity of the denser layers of the gas inwards. For Sun, its temperature is about 5,8000 C and the values of pressure & density are low.

Planet Swallowing: A process by which planets are diverted on to collision courses with their central stars and end up being destroyed by them.

Planisphere (Star Wheel): A device that can be adjusted to show the appearance of the night sky for any time and date on a round star map. Planispheres can be used to identify stars and constellations but not the planets, whose positions are always changing.

Photoelectric effect: The emission of an electron from the surface of a material when it is subjected to an EM wave of suitable frequency.

Plutino: A trans-Neptunian object with 2:3 mean motion resonance with Neptune. This simply means that for every 2 orbits made by a plutino, Neptune orbits 3 times.

Plasma: Plasma is one of the 4 fundamental states of matter. It is a low-density gas in which some of the individual atoms or molecules are ionized. The presence of so many charge carriers makes it electrically conductive so that it responds strongly to the electromagnetic field.

PP- Chain: PP Chain stands for Proton-Proton Chain. It is the nuclear process through which the Sun and Sun-like stars burn hydrogen. So what happens is 2 protons (H) fuse to form deuterium ( 2H ) and in the next process, a proton (H)fuses with (2H) to form 3He. These two process repeat and form another 3He and the two 3He fuse to form 4He, the helium atom. Gamma rays and neutrinos are also released.

Prism: It is the dispersive component of the astronomical spectrograph. It splits light into its constituent colors using the principle of refraction.

Prominences: A prominence is a large, bright, gaseous feature extending outward from the Sun’s surface, often in a loop shape. Prominences are anchored to the Sun’s surface in the photosphere and extend outwards into the Sun’s corona.

Proton Decay: The proton decay is a hypothetical decay in which a proton decays into lighter particles such as positron and muon. But there is no experimental evidence of the same. The theoretical half-life of a proton is 8.2 X 1O33 years and is predicted by Grand Unified Theory, the same theory which predicts the existence of magnetic monopoles.

Pulsars: A celestial object, thought to be a rapidly rotating neutron star, that emits regular pulses of radio waves and other electromagnetic radiation at rates of up to one thousand pulses per second.


Quadrature: The term quadrature in astronomy means the configuration in which the moon or a The planet lies at an angle of 900 east or west of Sun as seen from Earth. A planet can be in eastern or western quadrature depending on its position with respect to the Sun. Obviously, Mercury and-Venus cannot be at quadrature configuration. Only superior planets and the Moon can make an angle of 900.

Quantum Entanglement: A phenomenon that occurs when pairs or groups of particles are generated, interact, or share spatial proximity in ways such that the quantum state of each particle cannot be described independently of the state of the others, even when the particles are separated by a large distance.

Quarks: A quark is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. They are of 6 types.

Quark Star: A quark star is a hypothetical type of compact exotic star, where extremely high temperature and pressure has forced nuclear particles to form a continuous state of matter that consists primarily of free quarks.

Quasar: A quasar is an extremely luminous active galactic nucleus. It has been theorized that most large galaxies contain a supermassive central black hole with mass ranging from millions to billions of times the mass of the Sun. In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk.


Radial Velocity: The radial velocity of an object with respect to a given point is the rate of change of the distance between the object and the point. In astronomy, the point is usually taken to be the observer on Earth, so the radial velocity then denotes the speed with which the object moves away from or approaches the Earth.

Radiation Zone: There are 3 main regions inside a star. The core, the radiation zone, and the convection zone. Now the radiation zone lies next to the core. Here the energy is transported from the core through radiation in the form of photons, and not by convection. The plasma here is very dense: It takes 171,000 years for gamma-ray photons to cross the radiation zone. Convection and radiation zones are divided by Tachocline.

Redshift: Redshift happens when light emitted from an object is shifted to the red end of the spectrum, i.e the wavelength increases. This happens whenever a light source moves away from the observer. In astronomy, redshift is one of the most important evidence pointing towards the expansion of the universe.

Reflector: A telescope that gathers light with a mirror. The Newtonian reflector, designed by Isaac Newton, has a small second mirror mounted diagonally near the front of the tube to divert the light sideways and out to your eye.

Relativistic Beaming: Also known as the headlight effect, is the process by which relativistic effects modify the apparent luminosity of emitting matter that is moving at speeds close to the speed of light. It is a combination of time dilation and the Doppler effect.

Refractor: A telescope that gathers light with a lens. The original design showed dramatic rainbows, or “false color,” around stars and planets. Most modern refractors are achromatic, meaning “free of false color,” but this design still shows thin violet fringes around the brightest objects. The finest refractors produced today are apochromatic, meaning “beyond achromatic.” They use expensive, exotic kinds of glass to reduce false color to nearly undetectable levels.

Retrograde: When an object moves in the reverse sense of “normal” motion. For example, most bodies in the solar system revolve around the Sun and rotate counterclockwise as seen from above (north of) Earth’s orbit; those that orbit or spin clockwise have retrograde motion. This term also describes the period when a planet or asteroid appears to backtrack in the sky because of the changing viewing perspective caused by Earth’s orbital motion.

Right Ascension (R.A.): The celestial equivalent of longitude, denoting how far (in 15°-wide “hours”) an object lies east of the Sun’s location during the March equinox.

Roche lobe: The Roche lobe is the region around a star in a binary star within which orbiting material is gravitationally bound to that star. It is an approximately tear-drop-shaped region bounded by a critical gravitational equipotential, with the apex of the teardrop pointing towards the other star.

Runaway stars: A runaway star is one that is moving through space with an abnormally high velocity relative to the surrounding interstellar medium. The proper motion of a runaway star often points exactly away from a stellar association, of which the star was formerly a member before it was hurled out.


Scattered Disk: It is a circumstellar disk of distant icy minor planets beyond Neptune. The objects in this disks, SDOs, have highly eccentric orbits and inclination as high as 40o with the ecliptic.

Schwarzschild radius: The Schwarzschild radius is a physical parameter that corresponds to the radius defining the event horizon of a Schwarzschild black hole. It is a characteristic radius associated with every quantity of mass.

Scintillation: Also known as twinkling, Scintillation is the variation in a star’s brightness, wavelength and mean position as the light passes through the turbulent atmosphere.

Seeing: A measure of the atmosphere’s stability. Poor seeing makes objects waiver or blur when viewed in a telescope at high magnification. The best seeing often occurs on hazy nights, when the sky’s transparency is poor.

Singularity: A place in space-time where many physical quantities blow up. It must be noted that singularities do not exist in the real world. Although there exists a singularity in a black hole but recent studies and the observations indicate that even black hole formation is speculative.

Soft Gamma Repeater: A Soft Gamma Repeater (SGR) is a type of magnetar that emits bursts of soft or low energy gamma rays, each typically lasting less than a second, at irregular intervals.

Sol: The term sol is used by planetary astronomers to refer to the duration of a solar day on Mars. A mean Martian solar day, or “sol”, is 24 hours, 39 minutes, and 35.244 seconds and is often used as a direct replacement for “Day” when concerning Mars.

Solar Constant: It is the rate at which the solar electromagnetic radiation is received per unit area at the top level of the earth’s atmosphere. Each planet has a different solar constant due to varying distances from the sun.

Solar Cycle: The solar cycle or solar magnetic activity cycle is the nearly periodic 11-year change in the Sun’s activity (including changes in the levels of solar radiation and ejection of solar material) and appearance.

Solar Filter: Material that allows safe viewing of the Sun by blocking nearly all of its light. Proper filters should completely cover the front aperture of a telescope and should never be attached to the eyepiece; they range from the glass used by welders to the special plastic film. White-light filters will show sunspots, while hydrogen-alpha (H?) filters let certain red light through that reveals the Sun’s streaming hot gases.

Solar Flare: It is a sudden and dramatic release of energy through a break in Sun’s chromosphere, in the region of a sunspot. Energies equivalent to 1 billion megatons of TNT can be produced by a typical solar flare.

Solar Mass: The solar mass ( M ?) is a standard unit of mass in astronomy, equal to approximately 2×1030 kg. It is used to indicate the masses of other stars, as well as clusters, nebulae, and galaxies.

Solstice: The two times each year, around June 20th and December 21st, when the Sun is farthest north or south in the sky. At the summer solstice, the day is longest and the night is shortest, and vice versa at the winter solstice.

Space-time Singularity: A Space-time singularity is a location in space-time where the gravitational field of a celestial body is predicted to become infinite by general relativity in a way that does not depend on the coordinate system.

Spörer’s law: It predicts the variation of sunspot latitudes during a solar cycle. It was discovered by the English astronomer Richard Christopher Carrington around 1861. … As the cycle progresses, sunspots appear at lower and lower latitudes, until they average 15° at solar maximum.

Star: A massive ball of gas that generates prodigious amounts of energy (including light) from nuclear fusion in its hot, dense core. The Sun is a star.

Starburst Galaxy: A starburst galaxy is a galaxy undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies.

Star Cluster: A collection of stars orbiting a common center of mass. Open clusters typically contain a few hundred stars and maybe only 100 million years old or even less. Globular clusters may contain up to a million stars, and most are at least 10 billion years old (almost as old as the universe itself).

Star Diagonal: A mirror or prism in an elbow-shaped housing that attaches to the focuser of a refractor or compound telescope. It lets you look horizontally into the eyepiece when the telescope is pointed directly overhead.

Star Party: A group of people who get together to view the night sky. Astronomy clubs often hold star parties to introduce stargazing to the public.

Star Quakes: The equivalent of an earthquake in neutron stars is known as a starquake. Starquakes are believed to occur due to readjustment of star’s crust to regain the spherical shape. The largest starquake was detected on 27th December 2004. It was equivalent to 32 on Richter scale releasing energy over 1.85×1039 J. Had it occurred within 10 light-years from Earth, the quake would have resulted in mass extinction.

Steller Core: The stellar core is the innermost region of the stars where the fusion reaction takes place and is the hottest region of the star.

Stellar Aberration: A phenomenon which produces a difference between the actual position of a star and its observed position.

Sub-Giant Stars: When a star has finished fusing hydrogen in its core, the core is made of helium. But the hydrogen is still present in the shell around the inert core. Now, this star will increase in size to become a red giant and such a star is known as the subgiant star. Meanwhile, the temperature in the core shoots up due to its inactivity. As soon as the temperature of the core crosses the 180 million K mark, the core will reignite explosively and this time it will fuse helium into carbon.

Sunspot: A temporary dark blemish on the surface of the Sun that is a planet-size region of the gas cooler than its surroundings. Sunspots can be viewed safely using a solar filter.

Supernova: A star ending its life in a huge explosion. In comparison, a nova is a star that explosively sheds its outer layers without destroying itself.

Syzygy: In Astronomy, a syzygy is a straight-line configuration of three celestial bodies in a gravitational system. So all the eclipses, transits and occultations all occur at the time of a syzygy.


T Tauri Star: T Tauri stars are the type of variable stars that change their brightness periodically. These stars are quite young. In fact, they have not even started nuclear fusion in their core and often known as pre-main sequence star.

Tachocline: Tachocline is the transition region of the Sun, between the radiation zone and the convection zone. Due to different modes of rotation of the two zones, Tachocline is under tremendous shear and is measured to be located within a radius of about 0.70 times the radius of the Sun.

Tachyons: In particle physics, tachyons are hypothetical particles that always travel faster than light. Also, the mass of the particles is said to be imaginary.

Tau: The tau, also called tauon, is an elementary particle similar to the electron, with negative electric charge and a half-integral spin of 1/2.

Terminator: The line on the Moon or a planet that divides the bright, sunlit part from the part in shadow. It’s usually the most exciting and detailed region of the Moon to view through a telescope.

Thermal Runaway: Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result.

Time Dilation: A phenomenon in physics which tells that a clock in a moving frame of reference ticks slower than the clock which is stationary.

Transit: When Mercury or Venus crosses the disk of the Sun, making the planet visible as a black dot in silhouette. Or when a moon passes across the face of its parent planet. Transit also refers to the instant when a celestial object crosses the meridian and thus is highest in the sky.

Transparency: A measure of the atmosphere’s clarity. It refers to how dark the sky is at night and how blue it is during the day. When transparency is high, you see the most stars. Yet crystal-clear nights with superb transparency often have poor seeing.

Triple Alpha Process: The triple-alpha process is a set of nuclear fusion reactions by which 3 He-4 nuclei (alpha particles) are transformed into carbon. This process is the main source of energy in Red Giants and Supergiants.

Twilight: The time after sunset or before sunrise when the sky is not fully dark. Astronomical twilight ends after sunset (and begins before sunrise) when the Sun is 18° below the horizon.

Type Ia Supernova: A type Ia supernova (read “type one-a”) is a type of supernova that occurs in binary systems. (two stars orbiting one another) In it, one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf.


Unit-Power Finder: A device for aiming your telescope that shows the sky as it appears to your unaided eye, without magnification. The simplest type is a pair of notches or circles that you line up with your target. Other versions use an LED to project a red dot or circle onto a viewing window.

Universal Time (UT): Greenwich Mean Time, expressed in the 24-hour system. For example, 23:00 UT is 7:00 p.m. Eastern Daylight Time (or 6:00 p.m. Eastern Standard Time). Astronomers use Universal Time to describe when celestial events happen independent of an observer’s time zone.


Variable Star: A star whose brightness changes over the course of days, weeks, months, or years.


Waning: The changing illumination of the Moon (or other bodies) over time. The Moon wanes, becoming less illuminated, between its full and new phases.

Waxing: The changing illumination of the Moon (or another body) over time. The Moon waxes, growing more illuminated, between its new and full phases.

White Holes: In general relativity, white holes are those regions of space and time which cannot be entered from outside. Light and matter can escape from them. They originated from a solution of Einstein’s field equations and are thought to be very unstable.

Wolf Rayet stars: Wolf–Rayet stars are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionized helium and highly ionized nitrogen or carbon.

World line: A path traced by an object in the 4 D space-time. The y-axis of the graph is time and the x-axis is the space coordinates.

Wormhole: In relativity, wormholes are the theoretical shortcuts in space and time that connects long distances. They exist in the solutions of Einstein’s field equations. The first type of wormhole was Schwarzschild wormhole. Such a wormhole would collapse quickly and doesn’t allow space-time travel.


X-Rays: X-rays make up X-radiation, a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers. Along with several medical uses, they have a wide range of applications in astronomy.

X-Ray Burster: X-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity. It peaks in the X-ray regime of the electromagnetic spectrum.


Yerkes classification System: The system is used in the study of astronomy. It classifies stars on the basis of their Luminosity. Rather than their surface temperatures as seen in the Harvard Classification system.

Ylem: In the Big Bang theory, Ylem is considered to be the primordial matter present before the big bang occurred. It is believed to be composed of High energy Neutron Particles at extreme temperature and densities.


Zeeman Effect: The splitting of spectral lines under the effect of the external magnetic field. It is used to calculate the magnetic field of the stars in astrophysics.

Zenith: The point in the sky that’s directly overhead.

Zodiac: Greek for “circle of animals”. It’s the set of constellations situated along the ecliptic in the sky, through which the Sun, Moon, and planets move.

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